Blood glucose levels in Vipera aspis show great variations, both between months during the annual cycle, and between males and females. The seasonal variations are clearly related to climatic conditions, and particularly air temperatures and possibilities for thermoregulation. The blood glucose levels of the snakes are very low during the hibernation period and high in summer. Nevertheless, climatic conditions alone cannot explain changes of blood glucose levels observed during the period when the vipers are active, particularly the peaks measured soon after emergence from hibernation (males in February, females in March). Testicular activation, vitellogenesis and digestion need high body temperatures for long periods, thus the blood glucose level in reptiles chould be the result of a balance between climatic and physiological conditions expressed by the thermoregulatory behaviour. Plasma glucose levels are higher when the body temperatures are high and when the vipers spend more time in the sun (e.g. males in February, females in March); however, hormonal factors probably play a role.